1. Field of the Invention
The present invention relates to a flame retardant resin composition. More particularly, the present invention is concerned with a flame retardant resin composition comprising a resin component (A) which is a resin mixture of an aromatic polycarbonate and a styrene polymer, an organopolysiloxane (B) and at least one inorganic metal salt (C) selected from the group consisting of inorganic alkali metal salts and inorganic alkaline earth metal salts. In the present invention, by the combined use of an organopolysiloxane and an inorganic metal salt, a resin composition which contains an inherently flammable styrene polymer can be imparted with flame retardancy.
2. Prior Art
Aromatic polycarbonates are known as engineering plastics which have excellent impact resistance, heat resistance and transparency, and have been widely used in various fields, especially in the fields of office automation machines, household electric appliances and data communication equipments. In these fields, there has been a demand for mechanical parts having a small thickness as well as complicated structures. In accordance with such a demand, in order to improve the melt fluidity so as to achieve high injection moldability with respect to an aromatic polycarbonate, it has frequently been practiced to add a styrene resin to an aromatic polycarbonate. As the styrene resin used for this purpose, a rubber-modified styrene resin is generally used from the viewpoint of the desired properties, such as impact resistance, of the resultant aromatic polycarbonate resin composition.
In the fields of those electric/electronic devices which are mentioned above, for assuring the high safety of products, a high level of flame retardancy is required. An aromatic polycarbonate has a limiting oxygen index (which is a criterion for the flame retardancy) of from 26 to 27, and is known as a resin having a self-extinguishing property. The reason for this can be explained as follows. The main chain of the molecule of an aromatic polycarbonate is comprised mainly of aromatic rings. Therefore, when the aromatic polycarbonate is on fire, not only a thermal decomposition reaction but also a rearrangement reaction, a cyclization reaction and a crosslinking reaction occur, thereby easily forming a char layer which has extremely high flame retardancy.
On the contrary, a styrene polymer is a flammable resin having a limiting oxygen index as low as 18 and it is extremely difficult to inhibit the combustion thereof. The reason for this can be explained as follows. A styrene polymer is depolymerized by the combustion, and the resultant monomer acts as a fuel, causing the combustion to continue and also proceed acceleratedly. Consequently, in a resin mixture of an aromatic polycarbonate and a styrene polymer, as the amount of the styrene polymer is increased for improving the moldability, it becomes difficult to achieve high flame retardancy.
As a technique for imparting a high level of flame retardancy to a resin, it is known to incorporate thereinto flame retardants, for example halogen-containing flame retardants and auxiliary flame retardants, such as antimony oxide. However, in recent years, due to the growing interest in the environmental protection, studies have been made on the change of the types of flame retardants to those flame retardants which have higher safety.
For imparting flame retardancy to an aromatic polycarbonate and a resin mixture of an aromatic polycarbonate and a styrene polymer, a phosphate compound is generally employed as a flame retardant. However, a phosphate compound has a problem in that it is susceptible to hydrolysis and thermal decomposition and the resultant phosphoric acid causes a lowering of the molecular weight of the polycarbonate, leading to a lowering of the mechanical properties of the resin composition. Therefore, there arises a demand for a safer and more reliable flame retardant.
An organopolysiloxane is one of the non-halogen flame retardants, which has long been studied for its high safety.
In general, an organopolysiloxane is a polymer comprising recurring units of at least one type of unit selected from the group consisting of the following units:
a monofunctional siloxane unit (M unit) represented by the following formula (1): 
a bifunctional recurring siloxane unit (D unit) represented by the following formula (2): 
a trifunctional recurring siloxane unit (T unit) represented by the following formula (3): 
(in formulae (1) to (3), R is a monovalent organic group); and
a tetrafunctional recurring siloxane unit (Q unit) represented by the following formula (4): 
(see xe2x80x9cSilicone Handbookxe2x80x9d, edited by Kunio Ito and published by The Nikkan Kogyo Shinbun Ltd., Japan (1990)).
For example, Unexamined Japanese Patent Application Laid-Open Specification Nos. Hei 10-139964 (corresponding to EP No. 0 829 521) and Hei 11-140294 (corresponding to German Patent No. 19850453) each disclose a technique of improving the flame retardancy of an aromatic polycarbonate by using as a flame retardant a branched organopolysiloxane comprised mainly of a D unit and a T unit and optionally further comprised of a Q unit.
However, the effect of the conventional organopolysiloxane to impart flame retardancy to a resin is unsatisfactory when it is employed solely and, therefore, various studies have been made on the techniques of using the organopolysiloxane in combination with other flame retardants.
Representative examples of flame retardants which are generally used in combination with an organopolysiloxane include organic metal salts, such as a metal salt of an aromatic sulfonic acid and a metal salt of a perfluoroalkane sulfonic acid. Further, for preventing the dripping of flaming particles from a resin when the resin is on fire, a fluorine-containing olefin resin, such as polytetrafluoroethylene is frequently used.
Metal salts of organic sulfonic acids (such as an aromatic sulfonic acid and a perfluoroalkane sulfonic acid), especially alkali metal salts of organic sulfonic acids and alkaline earth metal salts of organic sulfonic acids, exhibit the effect of imparting flame retardancy to an aromatic polycarbonate even when such a metal salt of an organic sulfonic acid is used only in a small amount (specifically not greater than 0.5 part by weight, relative to 100 parts by weight of an aromatic polycarbonate). Therefore, such metal salts of organic sulfonic acids have long been used as a flame retardant (see, for example, Examined Japanese Patent Application Publication Specification Nos. Sho 47-40445, Sho 57-43099 and Sho 57-43100). The flame retarding mechanism of metal salts of organic sulfonic acids is described, for example, in G. Montaudo et al., Journal of Polymer Science, Polymer Chemistry, Vol. 26, p. 2113 (1988). Specifically, the above-mentioned article states that a metal salt of an organic sulfonic acid accelerates the thermal decomposition of an aromatic polycarbonate to thereby promote the formation of a char layer, resulting in imparting flame retardancy to the aromatic polycarbonate.
For example, Unexamined Japanese Patent Application Laid-Open Specification Nos. Hei 6-306265 and Hei 6-336547 (corresponding to U.S. Pat. No. 5,449,710) disclose a method in which an organopolysiloxane comprised mainly of a D unit and an alkali metal salt or alkaline earth metal salt of a perfluoroalkane sulfonic acid are added to an aromatic polycarbonate.
Unexamined Japanese Patent Application Laid-Open Specification Nos. Hei 11-217494 (corresponding to EP No. 1 035 169), 2000-302961 (corresponding to International Application Publication No. WO2000/64976), 2000-226527, and 2001-200150 disclose techniques of incorporating into polycarbonate resins organopolysiloxanes having varied types of branched structures which are comprised mainly of a D unit, a T unit and a Q unit, together with a metal salt of an organic sulfonic acid and a fluorine-containing polymer.
Unexamined Japanese Patent Application Laid-Open Specification Nos. Hei 8-176425, Hei 11-222559 (corresponding to U.S. Pat. No. 6,184,312), Hei 11-263903 and 2001-26704 disclose a technique of using, as a flame retardant for an aromatic polycarbonate, combinations of organopolysiloxanes having varied types of structures and a metal salt of an organic sulfonic acid.
The resins used in all the above-mentioned prior art patent documents are only aromatic polycarbonates. None of the above-mentioned prior art patent documents has any specific description about a flame retardancy-improving effect on a resin mixture of an aromatic polycarbonate and a styrene polymer. As mentioned above, a styrene polymer is a flammable resin, and hence it has conventionally been impossible to impart high flame retardancy to a resin mixture of an aromatic polycarbonate and a styrene polymer.
With respect to a technique wherein a combination of an organopolysiloxane and a metal salt of an organic, sulfonic acid is used as a flame retardant for a resin mixture of an aromatic polycarbonate and a styrene polymer, examples of prior art patent documents which disclose such technique include International Application Publication No. WO99/40158, Unexamined Japanese Patent Application Laid-Open Specification Nos. 2000-159996, 2000-256566 and 2000-345045, International Application Publication No. WO2000/46299, and Unexamined Japanese Patent Application Laid-Open Specification No. 2001-72867. In the Working Examples of these patent documents, in order to realize high flame retardancy of a resin mixture of an aromatic polycarbonate and a styrene polymer, a metal salt of an organic sulfonic acid is used in an amount as large as 1 part by weight, relative to 100 parts by weight of the resin mixture. In this connection, it should be noted that, when such a large amount of a metal salt of an organic sulfonic acid is added to an aromatic polycarbonate, it is possible that the thermal stability of the aromatic polycarbonate becomes low, causing a problem in that, during the melt-kneading and molding of the resultant resin composition, a lowering of the molecular weight of the polycarbonate occurs, leading to a lowering of the mechanical properties of the resin composition. Further, as described below, in the studies of the present inventors it has been found that, when the amount of a metal salt of an organic sulfonic acid which is added to a resin mixture of an aromatic polycarbonate and a styrene polymer is decreased in order to avoid a lowering of the mechanical properties of the resin composition, a problem arises in that flame retardancy improvement is not simply lowered; rather, for the worse, the flammability of the resin composition is promoted.
On the other hand, with respect to a technique wherein a salt other than a metal salt of an organic sulfonic acid is used as a flame retardant for an aromatic polycarbonate, examples of such techniques include a method which uses an alkali metal halide (see, for example, Unexamined Japanese Patent Application Laid-Open Specification Nos. Sho 49-88944, Sho 49-107049, Sho 49-125464 and Sho 50-83448); a method which uses an alkali metal salt or alkaline earth metal salt of sulfurous acid, thiosulfuric acid, dithionous acid or pyrosulfurous acid (see, for example, Unexamined Japanese Patent Application Laid-Open Specification Nos. Sho 52-17557 and Sho 52-17558); and a method which uses a fluorine-containing complex salt, such as sodium hexafluoroaluminate or potassium hexafluorosilicate (see, for example, Japanese Patent Application Laid-Open Specification Nos. Sho 48-43751, Sho 53-88856, Sho 53-96055 and Sho 55-151057). All these methods are intended to impart flame retardancy to an aromatic polycarbonate solely employed, but not to a resin mixture of an aromatic polycarbonate and a styrene polymer. Further, it is noted that there has been no conventional technique wherein a salt other than a metal salt of an organic sulfonic acid is used in combination with an organopolysiloxane.
In this situation, the present inventors have made extensive and intensive studies with a view toward developing an aromatic polycarbonate resin composition which contains an inherently flammable styrene polymer as a part of its resin component, wherein the resin composition exhibits high flame retardancy in spite of the presence of the styrene polymer. As a result, it has been found that high flame retardancy can be imparted to a resin mixture of an aromatic polycarbonate and a styrene polymer by using a flame retardant comprising, in combination, an organopolysiloxane (having high safety) and at least one inorganic metal salt selected from the group consisting of inorganic alkali metal salts and inorganic alkaline earth metal salts, and that, in addition, high flame retardancy can be obtained even when such flame retardant is used only in a very small amount, as compared to the case where a conventional flame retardant is used which comprises a combination of an organopolysiloxane and a metal salt of an organic sulfonic acid. The present invention has been completed, based this novel finding.
Accordingly, a primary object of the present invention is to provide an aromatic polycarbonate resin composition which contains a styrene polymer (which inherently has high flammability) as a part of its resin component, wherein the resin composition exhibits high flame retardancy in spite of the presence of the styrene polymer.
The foregoing and other objects, features and advantages of the present invention will be apparent to those skilled in the art from the following description and appended claims.